This invention relates to a thermosetting resin composition having excellent latent hardening characteristics, which comprises an epoxy resin and a novel hardener. More particularly, it relates to an epoxy resin composition having a remarkably elongated pot-life at storage temperatures as well as having a rapid hardening characteristic when heated.
Various kinds of epoxy resins have been employed in many fields, such as electrical insulation, moldings, castings, adhesives, coatings, etc., since the resulting cured articles are excellent in electrical, mechanical and chemical properties.
In one of the typical epoxy resin compositions which contains an acid anhydride as a hardener or curing agent, it is necessary to cure the composition at a considerably high temperature for a long time because the hardening characteristics of the composition are not good at the hardening temperature, while the composition is relatively stable at the storage temperature thereof. In order to improve the hardening characteristics of a composition of this type, there have been provided epoxy resin compositions containing suitable accelerators, such as, for example, tertiary amines, quaternary ammonium salts or complex salts of organo-metallic compounds. It is, however, an important disadvantage that these compositions have a very poor storage stability even though the hardening characteristics may be improved by adding such accelerators thereto. As an epoxy resin composition of latent hardening characteristics, there have been provided in the art epoxy resin compositions containing complexes of boron compounds as an accelerator. Not only are the latent hardening characteristics of these compositions insufficient, but also the properties of the cured articles tend to be undesirably influenced because these accelerators have a considerably water-absorption property.
Almost all of the thermosetting resins known in the art are capable of being molded by injection molding, and thermosetting resin compositions suitable for injection molding have been well developed. Injection molding is an advantageous technique because it is possible to remarkably save molding time and to increase production efficiency therewith. Only in the case of epoxy resin compositions, however, has injection molding not succeeded because there have not been provided epoxy resin compositions applicable and suitable for injection molding.
As is well known in the art, the resin composition to be injection molded is supplied to a heating cylinder provided with the screw of an injection molding machine, and the composition is subjected to heat in the cylinder, thereby softening the composition. This is followed by injection of the composition through a nozzle of the cylinder. In the case of conventional epoxy resin compositions, it has been difficult to avoid gelation of the composition in the heated cylinder because the hardening reaction of the softened composition begins to take place. Although it may be possible to prepare stable molding compositions in the cylinder of an injection molding machine by the addition of a suitable accelerator, it is then not possible to perform the injection molding with high efficiency because in such a case the hardening characteristics of the composition are insufficient, and it takes a long molding time to sufficiently harden the composition injected into the mold.
Therefore, in the injection molding of epoxy resin compositions, the compositions must have a superior latent hardening characteristic as compared with compositions conventionally used for transfer molding, compression molding or the like. In other words, the high thermal stability of the composition softened in the heating cylinder must be accompanied by a rapid hardening characteristic in the mold.